The present invention relates to an improvement in check valves, and more particularly a check valve used in positive displacement subsurface pumps in an oil production string, and more specifically to the valve seat thereof.
High pressure ball and seat type valves are commonly used in fluid transfer applications utilizing a metal-to-metal seat. Various fluid characteristics directly affect mechanical efficiency of this type seal. Given the nature of this type seal, premature failure often occurs, intermittent or continuous. Intermittent failure is often caused by suspended solids in fluid and/or entrained gases. Solids that become trapped between ball and seat prevent complete seal, and in high pressure applications cause pressure drop and extreme cavitation across face of the seat. Repeated intermittent failure will result in permanent seat damage and continuous valve failure. Although valve may not experience significant loss of mechanical efficiency with minute damage, the condition is irreversible and progressive. Given the nature of this type valve and application, pressure drop naturally occurs at the seating area. Entrained gases tend to break out as a result. Noncompressible fluid transfers smoothly while compressible fluid does not. Compressible fluid at high pressure transfers violently, causing impingement of the seating surfaces. This reaction is detrimental to valve efficiency and cycle life.
Another type affliction associated with metal-to-metal intermittent seating is electrolysis. Whereas fluid and metal are adequate conductors of electricity, the opposing seal surfaces act as electrodes discharging minute electrochemical energy between them. This discharging creates a failure similar in appearance and effect to that of cavitation and/or corrosion.
To overcome these problems various check valve arrangements have been suggested. U.S. Pat. No. 4,781,213 issued to Kilayko describes a ball check valve primarily designed using an O-ring seat wherein the O-ring has an ID less than the diameter of the ball and an OD greater than the diameter of the aperture. Kilayko disclose a ball check valve body comprising a base and a housing securable together. The assembly comprises a subassembly providing a first surface; an assembly fluid conduit terminates at the first surface in a port of circular cross section for communication with a base fluid conduit in the base. Within the assembly fluid conduit, an O-ring seat is aligned with the port; an elastomeric O-ring is positioned in the seat. Spacer means, radially outward of the O-ring seat, extends generally away from the first surface and has a dimension adjacent the O-ring seat greater than the cross sectional diameter of the O-ring when unstressed. Such an arrangement may be suitable for a chemical pump, but would be somewhat unworkable in a subsurface pump.
U.S. Pat. No 4,086,936 to Vork describes a one-way check valve for use in a reciprocating pump. The invention overcomes some of the disadvantages of the prior art by including resilient seat member having a non-complimentary shape to the spherical ball, and further having a deformable conical shape to provide variable pressure equalization. The valve employs a metallic ball sized in relation to the conical seat surface for providing limited deformation of the seat under pressure. He describes a typical Urethra material for the conical seat with a compressive strength of 20,000 pounds per square inch (p.s.i.) and a compressive modulus of 0.04-0.90.times.10.sup.5 p.s.i. To take advantage of this invention, the existing downhole pumps would have to be modified substantially including removing from the production string and replacing the downhole pump completely or at least the ball valve.
U.S. Pat. No. 5,062,450 to Bailey discloses the use of elastomers to reduce the weight of the valve body.
U.S. Pat. No. 2,695,628 to Wheildon describes a check valve with the ball being made of alumina and the seat of carbon boride or mixtures with metal borides. The function of the valve is to provided a check valve which is highly resistant to corrosion and abrasion such as is encountered in oil well pumps.
U.S. Pat. No. 4,662,392 to Vadasz describes a check valve for subsurface positive displacement pumps used for pumping crude oil with a high degree of solids. The ball check valve comprises a valve seat and a valve ball wherein the valve seat is comprised of a hollow cylindrical body defining an orifice through which fluid passes wherein the hollow cylindrical body is provided with an insert at the orifice of the hollow cylindrical body which forms the valve seat. The insert is formed of a material having a hardness greater than the hardness of the material from which the valve ball is formed which material in turn is characterized by a high resiliency. It is preferred that the ball of the valve be formed of a material having a high resiliency and a modulus of elasticity of between 0.1.times.10.sup.6 p s i. to 15.times.10.sup.6 p.s.i. and a surface hardness of 600-1200 on the Vickers scale. The inventor prefers carbon base materials for the valve ball including carbon base materials such as polymeric materials sold under the mark TORLON and manufactured by Amoco. The insert according to Vadasz is formed of a material having a hardness greater than 1300 on the Vickers scale. Vadasz suggests suitable materials for the insert include ceramic materials or sintered materials, including carbide. Utilizing Vadasz' invention again would require removing existing subsurface pumps and replacing the valves or the entire pump.
As a result of the relative differences in hardness of the various parts of the ball check valve as set forth above, the cylindrical body which is formed of the softest material dissipates a substantial part of the force generated by the impact of the valve ball on the hard insert.